[SOLVED] List all environment variable names in busybox

List all environment variable names in busybox

Environment variables with multiline values may confuse env's output:

# export A="B
> C=D
> E=F"
# env
A=B
C=D
E=F
TERM=xterm
SHELL=/bin/bash
USER=root
MAIL=/var/mail/root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/root
LANG=en_US.UTF-8
PS1=\h:\w\$
SHLVL=1
HOME=/root
LOGNAME=root
_=/usr/bin/env

In this case I can not just use awk -F= to extract all names because it shows wrong names C and E:

# env | awk -F= '{print $1}'
A
C
E
TERM
SHELL
USER
MAIL
PATH
PWD
LANG
PS1
SHLVL
HOME
LOGNAME
_

Then I figured out that env supports flag -0 to end each output line with 0 byte rather than newline, so using sed I could cut off the values in bash:

# env -0 | sed -e ':a;N;$!ba;s/\([^=]\+\)=\([^\x00]*\)\x00/\1\n/g'
A
TERM
SHELL
USER
MAIL
PATH
PWD
LANG
PS1
SHLVL
HOME
LOGNAME
_

But BusyBox's version of env does not support flag -0. Is there another way to do it?

Solution

This is maybe not an elegant but working solution. It first extracts all possible names from env's output, then verifies each of them using shell's expansion ${parameter:+word}. And finally it removes duplicates, since the same variable name could be printed on several lines in env's output (as a real variable name and as a part of some other variable's multiline value):

env | awk -F= '/[a-zA-Z_][a-zA-Z_0-9]*=/ {
        if (!system("[ -n \"${" $1 "+y}\" ]")) print $1 }' | sort | uniq

PS: The | sort | uniq part can be also implemented in awk.